We have constructed and isolated a recombinant retrovirus containing the src gene of Harvey murine sarcoma virus (Ha-MuSV) and the thymidine kinase gene (TK) of Herpes simplex virus type 1 (HSV-1). The new viruses can induce focus formation on NIH 3T3 cells and convert NIH 3T3 (TK) cells into the TK+ phenotype by carrying into the TK cells the HSV-1-tk gene. In the TK+ phenotype by carrying into the TK cells the HSV-1-tk gene. In the TK+ transformants, HSV-1 specific thymidine kinase can be identified by immunoassays. Hybridizaton analysis indicates that the recombinant virus contains both the Ha-MuSV src sequence and the tk gene sequence in a single RNA species of approximately 4.9 kilobases. We conclude that retroviruses can be used as true vectors for genes other than genes that lead to oncogenesis. We have also lined the HSV-1 tk gene to the deleted Moloney murine leukemia virus (Mo-MuLV) genome in double recombinants. The mouse TK- cells can be transformed into the TK+ phenotype with much higher efficiency by transfection using recombinant DNA containing both the tk and the defective Mo-MuLV than by using the tk DNA alone.